Interleukin 9 (IL-9) is a cytokine secreted by TH2 lymphocytes that acts as a regulator of a variety of hematopoietic cells, stimulates cell proliferation and prevents apoptosis (1,2). It functions through the interleukin 9 receptor (IL9R), which activates different signal transducer and activator (STAT) proteins (3). The IL-9 gene has been identified as a candidate gene for asthma (4). Genetic studies on a mouse model of asthma demonstrated that this cytokine is a determining factor in the pathogenesis of bronchial hyperresponsiveness (5).